Assuming her boyfriend has access to the Internet, books and civilization, he can learn about diabetes himself. If he can't be arsed to even do a 0.3s google search about a debilitating disease his loved one is suffering from then why should OP even bother doing all that?
If he's that selfish lazy and stupid, maybe he's not worth your time. He can't google it? Before he decided not to talk to someone, or to think it'll infect him, why didn't he look it up? That's a big step (breaking up/no longer talking to someone) to take without first verifying the facts...sigh